1. Field of the Invention
This invention relates generally to directional drilling, and, more particularly, to determining a trajectory of a directional drill.
2. Description of the Related Art
Directional drilling involves the controlled drilling of a borehole from a first point to a second point. For example, directional drilling may be used to drill a borehole from a location on the surface of the earth to a location deep underground. For another example, directional drilling may be used to drill a borehole that begins at a first location on the earth's surface, travels underground for a selected distance, and the returns to the surface at a second location. Typically, in directional drilling, the second point is horizontally, or azimuthally, displaced from the first so that one cannot simply drill straight down from the surface.
In the past, high costs limited the use of directional drilling to oil field and mining applications, where the drilling costs could be offset by the value of the oil or minerals produced by the well or mine. However, improvements in drilling technology have reduced the costs of some directional drilling applications. In particular, so-called “trench-less” directional drilling, in which nearly horizontal boreholes are drilled just below the earth's surface, has become a cost-effective method of drilling boreholes for various utility services, such as telephone lines, electrical lines, water lines, sewer lines, and the like. Trench-less directional drilling is typically employed when surface level obstructions make traditional trenched utility access difficult or impossible. For example, trench-less directional drilling is often used to drill boreholes beneath rivers, roads, commercial and residential buildings, and the like.
The drill string used to convey the cutting element into the borehole may bend during the directional drilling process. The bending of the drill string may be intentional, such as when a drill operator changes the heading of the cutting element, e.g. a drill bit, which changes the trajectory of the drill string. For example, the drill operator may change the heading of the cutting element to avoid an obstruction or to correct the trajectory. Sometimes, bending occurs unintentionally. For example, changes in the media being drilled or unbalanced forces produced by the drilling process may cause the heading of the cutting element, and the trajectory of the drill string, to change unexpectedly. Consequently, the actual trajectory of the drill string may deviate from the expected trajectory. The drill string is not, however, visible to the drill operator during the trench-less directional drilling process. Thus, trench-less directional drilling processes typically include some method of determining the trajectory of the drill string from the earth's surface.
Two methods are traditionally used to determine the trajectory of the directional drill. The first method uses a magnetometer package including an inclinometer and a magnetometer. The inclinometer is used to determine the inclination or tilt of the drilling assembly by measuring the earth's gravity vector and the magnetometer is used to determine a compass, or magnetometer, heading. The inclination and heading are measured at known depth positions and used to calculate the trajectory of the drill string. The second method uses a gyro package that includes a gyro and an inclinometer, which are similarly used to measure heading and inclination to calculate the trajectory of the drill string. However, the high cost of survey-accuracy magnetometer or gyro packages has restricted their use in trench-less directional drilling as discussed above. Furthermore, magnetic materials are often used in directional drilling assemblies, which may limit, or prevent, the use of magnetometers.
In recent years, another method has been employed to determine the trajectory using an electromagnetic signal. Typically, the electromagnetic signal is either injected or generated near the directional drilling assembly. An operator on the surface then detects the electromagnetic signal using an electromagnetic measurement device. The operator then uses the detected signal to determine the location of the drilling assembly. However, this method is limited to near-surface trench-less directional drilling applications. Furthermore, surface obstructions may make it difficult, or impossible, for the operator to detect the electromagnetic signal.
A laser positioning apparatus for an earth boring apparatus is described in Tull, et al (U.S. Pat. No. 5,361,854), hereinafter referred to as the Tull patent. In the Tull patent, a pair of lasers and a pair of light-sensing elements, or targets, are coupled to each of a plurality of sections of drill pipe. The angular displacement of adjacent sections of the drill pipe may be determined using oppositely directed laser beams produced by the lasers and detected by the light sensing elements. However, the use of multiple lasers and light-sensing elements may be limited by the cost of these devices. Furthermore, exposing sophisticated lasers and light-sensing elements to the borehole environment would likely result in high maintenance and repair costs, as well as diminished reliability.